U.S. Pat. No. 5,433,054 describes an inner frame for a cover and an outer frame for an inspection opening formed with frame elements having flanges projecting from the plane of the frame and formed with tongues at their ends engageable by resilient hooks of the respective corner elements which fit without play against the frame elements. The frame ends are cut at right angles to the longitudinal edges of the frame element in the case of the outer frame and are provided with abutting miters in the case of the inner frame.
The essential disadvantage of this corner connection is that the dimensional stability of the corners depends both on the accurate manufacture of the corner-piece/catch-tongue pairing and on the exact abutment of adjacent frame elements. This necessitates high manufacturing accuracies during the production of the corner connectors and the production of the frame elements.